Examples of the types of inks that can be used in inkjet printers include wax inks that are solid at normal temperatures, solvent-based inks containing an organic solvent as the main component, water-based inks containing water as the main component, and photocurable inks that are cured by light irradiation. Solvent-based inks are widely used in industrial applications such as outdoor advertising.
Inks for outdoor advertising applications not only require superior weather resistance for the ink coating film, but also require high gloss, a wide color gamut and superior alcohol resistance for the ink coating film, and superior stability and the like for the ink. The gloss and color gamut are strongly affected by the pigment dispersion level, and therefore in order to achieve the desired qualities, the dispersion time has needed to be lengthened, resulting in a problem of reduced productivity.
Furthermore, in order to improve the alcohol resistance of the ink coating film, the proportion of resin contained within the ink must be increased as much as possible. On the other hand, if the viscosity of the pigment dispersion is too high, then there is a possibility that the proportion of resin required to satisfactorily improve the alcohol resistance may not be achievable, meaning it is necessary to reduce the viscosity of the pigment dispersion.
In inkjet color printing, a three-color ink set composed of pigment inks of yellow (Y), magenta (M) and cyan (C), or a four-color ink set which also includes black (K), is often used to generate different hues.
In this case, the coloration vividness of mixed color portions containing two or more colors plays an important role in expanding the color reproducibility range.
Mixed color portions of two or more colors tend to suffer from reduced saturation. As a result, one method which could be considered would be to increase the pigment concentration in the ink, but this method tends to cause an increase in the ink viscosity. If the ink viscosity is too high, then the inkjet dischargeability tends to deteriorate, and as mentioned above, improving the resistance of the ink coating film by increasing the proportion of resin becomes impossible.
Accordingly, reducing the viscosity of the pigment dispersion is also necessary from the viewpoint of improving two-color coloration vividness.
In order to reduce the viscosity of the pigment dispersion, a pigment dispersant obtained by reacting a polyallylamine with a vinyl copolymer having a functional group that exhibits reactivity with an amino group has been proposed (Patent Document 1). Further, the use of a prescribed solvent system containing a glycol ether or the like has also been proposed (Patent Document 2).